To mount a tubeless automobile tire on a rim assembly, normally the inner peripheral annular- "beads" of the sidewalls of the tire are slipped over the circumference of one of the flanges of the rim assembly in a fashion similar to a button hole being slipped over a button, by changing the normally circular shape of the beads to an elongated oval shape and slipping the beads over one of the flanges of the rim assembly. With the side walls of the tire loosely positioned between the flanges and about the rim assembly, the next steps usually are: 1) to produce an air seal between the annular beads of the tire and the flanges of the wheel; 2) to "seat" the beads of the sidewalls of the tire against the flanges of the rim assembly to create a sturdy tire/wheel structure; and 3) to inflate the tire to a final inflation pressure suitable for use on the road.
As mounting of tires on rim assemblies can be laborious and time consuming, machines have been developed in the art to perform the step of slipping the tire over the rim assembly and inflating the tire. These machines include an air fill line for inflating the tire and a foot valve for delivering air to the air fill line. To seal a tire loosely positioned on a wheel with such a machine, an operator typically leans over the tire and rim assembly mounted on the machine and jostles the tire while depressing the foot pedal, causing pressurized air to flow through the tire inflator ring and through the air fill line, through the valve stem of the rim and into the tire. Usually this causes the tire to seal against the flanges of the rim. The operator then continues to depress the foot switch to use increasing air pressure within the tire to seat the beads.
It sometimes occurs that the tire beads resist seating against the flanges. This can result for example from a rusty or dirty wheel, an improperly shaped or sized rim or tire, or a failure of the operator to apply a proper lubricant to the tire prior to mounting. It is then common, though not recommended, for the operator to try to overcome this resistance by adding additional air to the tire to force the beads to seat against the flanges of the rim. Automobile tires are typically inflated to an air pressure of from 25 to 45 PSI for road use and this range is widely regarded as a safe pressure to attain during sealing and seating. The air compressors used in shops and garages to supply air to a tire changing machine typically are capable of delivering 120 PSI to 200 PSI of air pressure. In an attempt to seat the tire beads, an operator can improperly overinflate the tire to a dangerously excessive degree. This excessive air pressure can lead to a catastrophic failure of the tire bead or beads, particularly during the interval in which the tire bead yields to the increasing pressure and moves rapidly toward the seated position on the flanges. The catastrophic failure of the tire can release a large amount of energy and can result in a displacement of the tire and the rim assembly with great force and velocity. Injuries and even deaths have resulted from such a failure, by the rapidly displaced tire and wheel striking the head and upper body of the operator.
Automobile tires and rim assemblies typically have been designed with the tire bead diameter in whole inch sizes, e.g., 13 inch, 14 inch, 15 inch, etc. However, in recent years rim assemblies and tires also have been designed with tire bead diameters in half inch sizes, e.g., 15.5, 16.5 inch tire bead diameters. This has added a risk to safely mounting and inflating tires on rim assemblies because the half inch sizes make it possible to place a tire on the wrong size wheel assembly, e.g. a 16 inch tire on a 16.5 inch rim assembly. For example, some 16.5 inch rim assemblies have the same approximate maximum outside rim diameter as 16 inch rim assemblies, but have a larger bead seat diameter. FIG. 2 illustrates that the bead seat diameter, designated BSD in the figure, is smaller than the outside diameter of the flange. Thus, a 16 inch tire will slip over some 16.5 inch flanges, but it is usually impossible to bead seat a 16 inch tire on the 16.5 inch rim assembly with the use of bead seating normal air pressures of about 15 PSI. This tends to increase the adverse likelihood that the operator may over-pressurize the tire in an attempt to bead seat the tire.
Efforts have been made in the past to minimize the danger of catastrophic tire failure due to over-pressurization of the tire during bead seating of the tire against the flange of its rim assembly by limiting the pressure of the source air supplied to the air fill line in order to prevent the tire from receiving excessive air pressure. This has the distinct disadvantage of reducing the rate of air flow into the tire and therefore increasing the time it takes to fill the tire to the proper operating air pressure. This is due to the reduced pressure differential between the source air pressure in the filling hose and the air pressure within the unfilled tire. Of course, as the tire begins to fill and the pressure within the tire approaches the source air pressure, the pressure differential is made smaller yet, slowing the delivery of air to the tire even more. A reduction in source air pressure, with a corresponding reduction in flow volume through the fill line, also makes it difficult to seal the tire against the rim flanges. The operators of such equipment may improperly circumvent such air pressure limiting devices so as to use source higher air pressures and faster air flow rates to seal and more quickly fill the tires, leading to increased hazards of tire rupture as described above if the developed air pressures in the tires are not properly monitored by the operator.